I. Field of the Invention
The present invention is directed to a proximity sensing arrangement employed as a control in a machine for manufacturing laminated cores for dynamo-electric machines.
II. Description of the Prior Art
Lamination stamping machines for which the proximity sensing apparatus of the present invention is designed are generally well known in the prior art. Generally speaking, such machines take the form of a progressive die through which a strip of sheet metal stock is advanced through successive die stations at which various openings, apertures and tab-like projections are formed, and then to a final station where the laminations are punched from the stock into a rotary lamination receiving die. Typical examples of such prior art apparatus are shown in U.S. Pat. Nos. to Mitsui 4,110,895; Wurth 4,578,853; and Oboshi et al. 4,597,168.
In most induction motor rotors, it is necessary that each lamination within the assembled rotor core be rotatively offset about the rotor axis from the adjacent lamination. This rotative offset produces what is referred to as a skew angle in the assembled core. The apparatus disclosed in the above-identified Mitsui U.S. Pat. No. 4,110,895, as well as other prior art machines, accomplishes the desired rotational offset between adjacent lamina by feeding the lamina as punched from the sheet metal stock into a tightfitting bore in a rotatable die. A mechanical linkage actuated by downward movement of the punch rotates the rotatable die through the desired angular increment as the punch descends.
In addition to rotating the rotatable die prior to the addition of each lamina to the stack of lamina being built up in the die to achieve the desired skew angle, rotation of the stack may also be performed to compensate for variations in thickness in the metal stock from which the laminations are formed. Such variations in thickness can occur both longitudinally and transversely of the strip of sheet metal stock. If such variations are severe, and the thicker edges of successive lamina are stacked upon each other, an unsymmetrical rotor will be produced. If, however, the stack is rotated 180.degree. prior to the addition of each lamina to the stack, the thickness variations will be largely canceled out. This problem has been addressed by the prior art; see, for example, Bergmann U.S. Pat. No. 3,823,460 and Neuenschwander U.S. Pat. No. 4,619,028. In recent years, specialized electric drive motors capable of rapid and precise, intermittent, incremental, rotative steps have become available for accomplishing such die rotation. These motors, combined with computerized control systems show great promise and offer many advantages over mechanical arrangement, such as that of U.S. Pat. No. 4,110,895 referred to above.
However, desired rotor core production rates require high-speed operation of the stamping machine and production goals of 300 strokes per minute pose problems, particularly if, within a stroke, the die must be rotated both to set skew angle and to compensate for thickness variation. At 300 strokes per minute, a single stroke is completed every 0.2 seconds (200 milliseconds). For a certain portion of this time period, the punch will be projecting into or through the sheet metal stock and certain manipulative steps which must be performed within a stroke cycle cannot be commenced until the punch is retracted clear of the metal strip.
Presently available proximity switches or proximity sensors may be employed to accurately signal when the rapidly moving punch is at any selected position within its cycle. Such proximity sensors are well adapted to produce a control signal when the punch is at a precisely selected location in its cycle; however, the initial setup time increases with the degree of precision desired and a precise setting which will result in the triggering of the proximity switch when the punch is at a precisely selected position within a stroke cycle measured in milliseconds can require several hours. Because the die surface must be ground periodically to sharpen the die edges, usage of the die surface as a reference point for the location of the proximmity switch triggering point will ordinarily require a complete repetition of the initial setup procedure for the switch each time the die is sharpened.
The present invention is especially directed to a proximity switch arrangement which may be precisely set with reference to the die surface and does not require resetting upon sharpening of the die.